Well, this is all entirely my opinion...
For me, I would say I prefer the Playstation 3, reasons why?
- Free online gaming
I wasn't too keen on buying the Xbox due to having to pay an annual fee of up to £40 to play online. At first the Xbox seems the cheaper option, but then the online fees and other accessories soon mount up. Though the new Xbox SLim has built in Wi-Fi and a 250GB HDD, so that's a better deal compared to its previous models. I'm not sure if the Xbox's online is more stable, as most games use P2P (Pee-to-Peer) connections for their online games, so the stability is down to whoever is hosting the session. Though Xbox's online does have Cross Game Chat (the ability to talk to your friend via headsets even when playing other games). This sounds like a cool addition.
- PS3 Exclusive games
Uncharted, Killzone, Ratchet and Clank, Resistance, Gran Turismo, Tekken, Metal Gear Solid, inFamous, LittleBigPlanet, Modnation... all very good games, I throoughly enjoy the majority of them.
- PS Move
This will turn the PS3 into a HD Wii. Yes there will now be 'casual' games on the PS3 but there will be no decline in more 'hardcore' games and such, it looks awesome. Especially when you'll be able to play games like Killzone 3 with it, in 3D too (as long as you have a 3DTV)! Also, when compared to Xbox, it's the price thing again for me. PS Move can cost between £30-80, where as Xbox's Kinect will cost £130. Also, Kinect relies on hand/body movement only, as with the PS Move you have have physical controllers for shooter games etc. I'm not sure how you'd be able to do that with Kinect, unless they bring out a gun controller that'll be picked up by Kinect.
- It can also play Blu-ray discs
Blu-rays do look immense. When I saw The Dark Knight on BLu-ray, I was blown away.
- More secure
Xbox has been hacked, so favourited games such as Call of Duty has been exploited so people can change the rules of online lobbies or give themselves unlimited ammo etc.
The Wii... meh. All it really has now are its exclusives (Zelda, Pokemon, Metroid, Mario etc.).
Now having said all that, the Xbox is a good console, it's just when I personally weigh up the options, I prefer the PS3.